One way to get to a quiet supersonic transport sooner rather than later would be to design an aircraft flies at a combination of speed and altitude at which its shockwaves do not reach the ground because of atmospheric refraction.
That speed is around Mach 1.2 and, with the absence of a sonic boom allowing supersonic flights over land, would be usefully faster than today’s fastest civil aircraft – the Gulfstream G650 at Mach 0.925 and Cessna’s uprated Citation X at Mach 0.935.
Such an aircraft would exploit a phenomenon known as “Mach cut-off” where, for an aircraft flying at M1.2 or less at 35,000ft or above, the shockwaves do not reach the ground. Instead, as they enter warmer air closer to the ground and the speed of sound increases, they are refracted, or bent, upwards away from the ground.
But when the shockwaves curve upward, they create a series of sonic-boom waves focused along a line. As the diagram above shows, the sonic-boom waves are above this “caustic line”, but below it, on the “shadow side”, something called evanescent waves are generated that can propagate down to be heard by people to the ground.
According to NASA, these evanescent waves sound like distant thunder, and quickly fade with distance, but whether they would be judged loud enough to be perceived as unacceptable is not known for sure. So NASA is measuring the evanescent waves with a series of supersonic flights at Edwards AFB, Calif.
Under the Far-field Investigation of No Boom Threshold, or FaINT, project, the agency and its industry and university partners are using an array of microphones on the dry lake bed to collect data on the propagation of evanescent waves near the shadow side of both normal and loud sonic booms.
The data will help with the design of low-boom supersonic transports. Potential beneficiaries include Aerion, which has been exploring Mach cut-off flight as a way to increase the appeal of its low-drag but normal-boom supersonic business-jet design, by allowing it to fly overland at about M1.15.
Aerion, which has been trying to get the backing of a large OEM to build its Mach 1.6 design, now says there is interest in using its natural laminar-flow technology first to build a near-sonic business jet that could over time to developed into a low-supersonic aircraft capable of flying overland at Mach cut-off speeds.